A micro-grid is a localized grouping of electricity generation, energy storage, and loads that can be switchably connected to a conventional power distribution network (macro-grid). Generation and loads in the micro-grid are usually interconnected at low voltage. From the point of view of the grid operator, a connected micro-grid can be controlled as if it is one entity.
Micro-grids can improve power quality and reliability by organizing distributed generation (DG) units to provide power service locally. Therefore, micro-grids are mainly used to incorporate distributed energy resources to relieve power flows in current stressed power distribution networks. By combining generation, storage, and load devices, the micro-grid can either produce or consume electricity.
The micro-grid typically connects to a power distribution network through a single point of common coupling (PCC).
From the perspective of a power distribution network operator, the PCC can be either a generator bus, a load bus, or even disconnected when the micro-grid operates in stand-alone mode. As a load bus, if power consumption increases, then a voltage collapse can occur.
A power network enters a state of voltage instability when a change in system conditions causes an uncontrollable voltage decrease. Voltage instability is mainly caused by an inability of the power network to supply sufficient reactive power, such as in a stressed power network.
There is a growing concern about stressed power networks due to increasing electricity demand and an aging infrastructure. Furthermore, power distribution networks operate close to voltage stability limits when micro-grids are present, which complicate power flow. Because power distribution networks become more vulnerable to voltage collapse, distribution system operators need to detect and even predict an impending voltage collapse accurately and timely.
There are several methods available to assess static voltage stability in power networks, such as a critical load impedance method, a continuation power flow method, and a regular power flow based method.
The critical load impedance method predicts the voltage instability by measuring critical load impedances for load buses based on an equivalent circuit model derived using either model-based or measurement-based method.
U.S. Pat. No. 7,996,116 describes a model-based method that derives an equivalent circuit model by estimating states of the network. U.S. Pat. No. 8,126,667 describes a measurement-based method that decides a voltage stability margin based on synchronized phasor measurements from the entire power system. As an alternative, U.S. Pat. No. 5,745,368 describes a continuation power flow method that approximates a voltage versus power curve to determine the critical point. U.S. 20140222227 describes an improved continuation power flow method that claims to obtain a better approximation of the voltage versus power curve. There are also voltage stability detection methods that are based on power flow solutions. U.S. Pat. Nos. 4,974,140, 7,519,506 and U.S. Pat. No. 7,979,239 describe a procedure of examining power flow solutions using power flow analysis results of the entire power system.
For the above prior art methods, some level of approximation exists because required information is usually unavailable, or it takes time to obtain an accurate parameter estimation. Some of those methods use approximations to simplify the voltage stability prediction problem, such in continuation power flow methods.
Other methods require synchronized phasor measurements of the entire power network, but load buses generally do not have such equipment. In addition, solving parameter or state estimation problems for the entire power system takes time and may not converge. As a result, most of available methods cannot analyze real-time static voltage stability without requiring excessive information from a micro-grid-integrated power distribution network.